1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a first print time taken to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate a sufficient amount of heat even when a plurality of recording media is conveyed through the fixing device continuously at increased speed for high speed printing.
To address these requests, the fixing device may employ a thin endless belt having a decreased thermal capacity and therefore heated quickly by a heater. FIG. 1 illustrates such fixing device 20R1 that incorporates a thin endless belt 100. For example, as shown in FIG. 1, a pressing roller 400 is pressed against a substantially tubular, metal thermal conductor 200 disposed inside a loop formed by the endless belt 100 to form a fixing nip N between the pressing roller 400 and the endless belt 100. A heater 300 disposed inside the metal thermal conductor 200 heats the endless belt 100 via the metal thermal conductor 200. As the pressing roller 400 and the endless belt 100 rotate and convey a recording medium P bearing a toner image T through the fixing nip N in a recording medium conveyance direction A1, the endless belt 100 and the pressing roller 400 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P. Since the heater 300 heats the endless belt 100 via the metal thermal conductor 200 that faces the entire inner circumferential surface of the endless belt 100, the endless belt 100 is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating heat sufficiently.
However, in order to shorten the first print time further and save more energy, the fixing device is requested to heat the endless belt 100 more efficiently. To address this request, a configuration to heat the endless belt 100 directly, not via the metal thermal conductor 200, is proposed as shown in FIG. 2.
FIG. 2 illustrates a fixing device 20R2 in which the heater 300 heats the endless belt 100 directly. Instead of the metal thermal conductor 200 depicted in FIG. 1, a nip formation plate 500 is disposed inside the loop formed by the endless belt 100 and presses against the pressing roller 400 via the endless belt 100 to form the fixing nip N between the endless belt 100 and the pressing roller 400. Since the nip formation plate 500 does not encircle the heater 300 unlike the metal thermal conductor 200 depicted in FIG. 1, the heater 300 heats the endless belt 100 directly, thus improving heating efficiency for heating the endless belt 100 and thereby shortening the first print time further and saving more energy.
On the other hand, as the endless belt 100 rotates and conveys the recording medium P, the endless belt 100 may be skewed in the axial direction thereof. To address this problem, a stationary flange 600 may be disposed at each lateral end 100a of the endless belt 100 in the axial direction thereof as shown in FIG. 3. As the endless belt 100 is skewed in the axial direction thereof, the lateral end 100a of the endless belt 100 in the axial direction thereof comes into contact with the flange 600 that restricts movement of the endless belt 100 in the axial direction thereof. However, as the lateral end 100a of the endless belt 100 comes into contact with the flange 600, it may be damaged by friction between the endless belt 100 and the stationary flange 600. To address this problem, a ring 700 may be interposed between the lateral end 100a of the endless belt 100 and the flange 600 to protect the lateral end 100a of the endless belt 100. For example, as the endless belt 100 is skewed in the axial direction thereof and the lateral end 100a of the endless belt 100 strikes the ring 700, the ring 700 rotates in accordance with rotation of the endless belt 100 with a reduced friction therebetween, thus minimizing damage and abrasion of the lateral end 100a of the endless belt 100.
However, the ring 700 is subject to deformation during assembly and operation. For example, if the ring 700 is deformed as it is attached between the endless belt 100 and the flange 600 or if the ring 700 is made of a low friction material that reduces friction between the ring 700 and the endless belt 100, the ring 700 is subject to plastic deformation that obstructs rotation of the ring 700 in accordance with rotation of the endless belt 100. Accordingly, the ring 700 may impose an increased load on the lateral end 100a of the endless belt 100, which may damage the lateral end 100a of the endless belt 100. Moreover, if the endless belt 100 deviates from its proper rotation locus and accidentally enters a through-hole of the ring 700, the ring 700 may damage the lateral end 100a of the endless belt 100.